Bacterial Infections

Bacterial infections are those caused by bacteria.

See below for articles about specific bacterial infections

About bacteria

Bacteria are single-celled microorganisms that are invisible to the naked eye. The singular form of the term is bacterium. Abundant in the air, soil, and water, most bacteria are harmless to humans. Some, such as those that live in the intestine, are beneficial and help to break down food for digestion.

Disease-causing types 

Disease-causing bacteria are known as pathogens and are classified, according to shape, into three main groups: cocci (spherical); bacilli (rod-shaped); and spirochaetes or spirilla (spiral-shaped). Among the wide range of diseases caused by cocci are pneumonia, tonsillitis, bacterial endocarditis (inflammation of the lining inside the heart), meningitis (inflammation of the membranes surrounding the brain and spinal cord), toxic shock syndrome, and various disorders of the skin. Diseases that are caused by bacilli include tuberculosis, pertussis (whooping cough), typhoid fever, diphtheria, tetanus, salmonellosis, shigellosis (bacillary dysentery), legionnaires’ disease, and botulism. Bacteria from the third, and smallest, group, the spirochaetes, are responsible for causing syphilis, yaws, leptospirosis, and Lyme disease.

Growth and movement

The bacteria that colonize the human body thrive in warm, moist conditions. Some of these bacteria are aerobic (they need oxygen to grow and multiply) and are therefore most commonly found on the skin or within the respiratory system. Others are anaerobic, thriving where there is no oxygen, deep within tissue or wounds. Some types of bacteria are naturally static; if they move around the body at all, they do so only when carried in currents of air or fluid. However, there are also highly motile types of bacteria, such as salmonella, which move through fluids by lashing with their whiplike tails (known as flagella) and can anchor themselves to other cells with filamentous threads called pili.


Bacteria reproduce by simple cell division, which can occur every few minutes in ideal conditions (exactly the right temperature and sufficient nourishment for all cells). Some bacteria multiply by each producing a spore (a single new bacterium). Spores, which are protected by a tough membrane, can survive high temperatures, dry conditions, and lack of nourishment.

How bacteria enter the body 

Bacteria can enter the body through the lungs if they are inhaled in infected droplets spread by coughs and sneezes. The digestive tract may become infected if contaminated food is eaten. Some bacteria cause diseases, such as sexually transmitted infections, by entering the genitourinary system. Bacteria can also penetrate the skin in various ways: through hair follicles; by way of superficial cuts and abrasions; through burns; and via deep, penetrating wounds.

How bacteria cause bacteria cause disease 

Some bacteria release poisons (toxins) that are harmful to human cells. The toxins either destroy the cell or disrupt its chemical processes. Less commonly, certain types of bacteria directly enter, and multiply within, body cells, causing tissue damage as they spread.

The body's defences 

The body’s first defences against disease- causing bacteria are the skin and the mucous membranes lining the respiratory tract, the digestive tract, and the genitourinary system. The eyes are protected by an enzyme in tears and the stomach secretes hydrochloric acid, which kills many of the bacteria found in food and water. If bacteria pass through these barriers, the body’s immune system responds by sending various types of white blood cell to seek and destroy the bacteria. Immunity can also be generated by immunization. This involves injecting a weakened form of the bacterium or its poison into the body to stimulate an immune response. Immunization is now routine for a number of conditions, including diphtheria, tetanus, and some forms of meningitis.

Treatment of bacterial diseases

The immune response is sometimes enough to bring about recovery, and mild bacterial infections may not need any treatment. However, antibiotic drugs are the main form of treatment for more severe infections. Superficial infected wounds may be treated with antiseptics. Some bacteria, such as MRSA, are now becoming resistant to treatment with antibiotics. In these circumstances, bacterial infections can be difficult or even impossible to treat and may be life threatening.

Read more: - detailed articles about specific bacterial infections:


Bacteraemia is the presence of bacteria in the bloodstream. Bacteraemia commonly occurs for a few hours after minor surgical operations and dental treatment and may also occur in infections such as tonsillitis. The immune system, the body’s natural defence mechanism, usually prevents the bacteria from multiplying and causing damage. However, in people with abnormal heart valves (due to conditions such as a congenital defect or scarring from rheumatic fever), the bacteria may cause endocarditis (inflammation of the heart lining and valves). If bacteraemia affects a person whose immune system is weakened by illness or major surgery, septicaemia (an infection of the blood) may develop.


Bacteremia occurs when bacteria enter the bloodstream. This may occur through a wound or infection, or through a surgical procedure or injection. Bacteremia may cause no symptoms and resolve without treatment, or it may produce fever and other symptoms of infection. In some cases, bacteremia leads to septic shock, a potentially life-threatening condition.

Risk factors

Any opening through the skin and/or body orifices that allows for the entrance of bacteria into the body places an individual, particularly those with a compromised immune system, at increased risk for the development of bacteremia.

Conditions that increase the chances of developing bacteremia include:

  • immune suppression, either due to HIV infection or drug therapy that suppresses the immune system
  • antibiotic therapy, which changes the balance of bacterial types in the body
  • prolonged or severe illness
  • alcoholism or other drug abuse
  • malnutrition
  • diseases or drug therapy that cause ulcers in the intestines, e.g., chemotherapy for cancer

Causes and symptoms


Several types of bacteria live on the surface of the skin or colonize the moist linings of the urinary tract, lower digestive tract, and other internal surfaces. These bacteria are normally harmless as long as they are kept in check by the body’s natural barriers and the immune system. People in good health with strong immune systems rarely develop bacteremia. However, when bacteria are introduced directly into the circulatory system, especially in a person who is ill or undergoing aggressive medical treatment, the immune system may not be able to cope with the invasion, and symptoms of bacteremia may develop. For this reason, bacteremia is most common in people who are already affected by or being treated for some other medical problem. In addition, medical treatment may bring a person in contact with new types of bacteria that are more invasive than those already residing in that person’s body, further increasing the likelihood of bacterial infection.

Common immediate causes of bacteremia include:

  • drainage of an abscess, including an abscessed tooth
  • urinary tract infection, especially in the presence of a bladder catheter
  • decubitus ulcers (pressure sores)
  • intravenous procedures using unsterilized needles, including IV drug use
  • prolonged IV needle placement
  • use of ostomytubes, including gastrostomy (surgically making a new opening into the stomach), jejunostomy (surgically making an opening from the abdominal wall into the jejunum), and colostomy (surgically creating an articifical opening into the colon)

The bacteria most likely to cause bacteremia include members of the Staphylococcus, Streptococcus, Pseudomonas, Haemophilus, and Esherichia (E. coli) genera.


Symptoms of bacteremia may include:

  • fever over 101F (38.3C)
  • chills malaise
  • abdominal pain
  • nausea
  • vomiting
  • diarrhea
  • anxiety
  • shortness of breath
  • confusion

Not all of these symptoms may be present. In the elderly, confusion may be the only prominent symptom. Bacteremia may lead to septic shock, whose symptoms include decreased consciousness, rapid heart and breathing rates, and multiple organ failures.



Bacteremia is diagnosed by culturing the blood for bacteria. Samples may need to be tested several times over several hours. Blood analysis may also reveal an elevated number of white blood cells.


Bacteremia may cause no symptoms, but may be discovered through a blood test for another condition. In this situation, it may not need to be treated, except in patients especially at risk for infection, such as those with heart valve defects or whose immune systems are suppressed.


Prompt antibiotic therapy usually succeeds in clearing bacteria from the bloodstream. Recurrence may indicate an undiscovered site of infection. Untreated bacteria in the blood may spread, causing infection of the heart (endocarditis or pericarditis) or infection of the covering of the central nervous system (meningitis). Blood pressure is monitored closely; a decline in blood pressure may indicate the onset of septic shock.


If detected and treated promptly, most individuals recover from bacteremia. However, in individuals whose immune systems are compromised, it is critical that the condition be treated promptly and aggressively so that it does not progress to sepsis or septic shock, which can lead to death even if treatment is initiated.


Bacteremia can be prevented by preventing the infections that often precede it. Good personal hygiene such as effective hand-washing, especially during viral illness, may reduce the risk of developing bacterial infection. Treating bacterial infections quickly and thoroughly can minimize the risk of spreading infection. During medical procedures, the burden falls on medical professionals to minimize the number and duration of invasive procedures, to reduce patients’ exposure to sources of bacteria when being treated, and to use scrupulous technique.